4. Bxc6 in Ruy Lopez

Why do most players rarely or perhaps never play 4. Bxc6? In my opinion, trading a good bishop with a knight is a disadvantage for the endgame. A light-squared bishop is crucial for a good win.

Below is a game I played earlier as black, where my opponnet played 4. Bxc6 in a Ruy Lopez Opening.

This is why I never played 4. Bxc6~